<?php
$html = '%5B%5Bsignature%5D%5D here';
echo str_replace('%5B%5Bsignature%5D%5D', 'replaced!', $html);
PD9waHAKCiRodG1sID0gJyU1QiU1QnNpZ25hdHVyZSU1RCU1RCBoZXJlJzsKZWNobyBzdHJfcmVwbGFjZSgnJTVCJTVCc2lnbmF0dXJlJTVEJTVEJywgJ3JlcGxhY2VkIScsICRodG1sKTs=